Preparation of alkylene carbonates
This Application is a division of Ser. No. 502,024 filed June 7, 1983, now U.S. Pat. No. 4,841,072 which was a continuation of Ser. No. 326,447 filed Dec. 2, 1981, now abandoned. 
    
    
     PRIOR ART 
     The invention relates to a process for the preparation of alkylene carbonates by the reaction of the corresponding alkylene oxide with carbon dioxide. Such reactions are well known in the art. Alkylene carbonates are useful as solvents or as a source of the corresponding glycols. They are of particular interest as intermediates in the process of converting ethylene oxide into ethylene glycol while avoiding the inefficiency associated with the conventional hydration process. 
     Several processes have been disclosed for a single step hydration of alkylene oxides to glycols in the presence of a catalyst and carbon dioxide. Such processes are said to make possible the reduction in the amount of water used. The removal of excess water is a major expense in the conventional hydration process. The carbon dioxide is not consumed in the process, but it has been suggested that the hydration proceeds via the alkylene carbonate as an intermediate compound. 
     U.S. Pat. No. 3,922,314 discloses a process for the hydration of ethylene oxide to ethylene glycol which uses no catalyst, but operates with an aqueous ethylene oxide solution containing at least 8 wt % ethylene oxide and at least 0.1 wt % carbon dioxide. 
     A catalytic process is described in British patent 1,177,877 (or U.S. Pat. No. 3,629,343). Alkylene oxides are hydrated to the glycols at temperatures of 80°-220° C. and pressures of 10-180 atmospheres in the presence of a halide catalyst. Preferred are alkali metal or quaternary ammonium halides, particularly bromides and iodides. Alkali metal hydroxides, carbonates, or bicarbonates were said to be beneficial. 
     A similar process is discussed in U.S. Pat. No. 4,160,116 where quaternary phosphonium halides, preferably the iodides and bromides were used to catalyze the hydration of alkylene oxides in the presence of carbon dioxide. The temperature is 50°-200° C. and the pressure 3-50 kg/cm 2 . 
     Still another such process is disclosed in published Japanese patent application 81-45426, in which molybdenum and/or tungsten compounds are combined with known catalysts such as alkali metal halides, quaternary ammonium or phosphonium salts, organic halides, and organic amines. The reaction is stated to be carried out at 20°-250° C. and 0-30 kg/cm 2  gauge. 
     The formation of alkylene carbonates, as opposed to the hydration of alkylene oxides to glycols, takes place in the prior art to be discussed with no water present. Catalysts and reaction conditions similar to those described above for the hydration of alkylene oxides have been disclosed to be useful. 
     In U.S. Pat. No. 2,667,497 magnesium or calcium halides were used at 150°-250° C. and 500-2000 psi to produce alkylene carbonates from the corresponding oxides. 
     U.S. Pat. No. 2,766,258 discloses the use of quaternary ammonium hydroxides, carbonates, and bicarbonates to catalyze the reaction of alkylene oxides with carbon dioxide. The reaction was carried out at temperatures between 100°-225° C. and pressures of 300-500 psig. 
     The quaternary ammonium halides were used by the patentees in U.S. Pat. No.2,773,070 at temperatures of 100°-225° C. and pressures greater than 800 psi. 
     Amines were the catalyst used for the reaction by the patentees in U.S. Pat. No. 2,773,881. The reaction was carried out at 100°-400° C. and more than 500 psi. 
     Three patents issued to the same assignee, i.e. U.S. Pat. Nos. 2,994,705; 2,994,704; and 2,993,908 disclose substantially the same conditions, 93°-260° C. and 8-212 kg/cm 2  gauge, with organic phosphonium halides, organic sulfonium halides, and urea hydrohalides given as catalysts for the preparation of alkylene carbonates from the corresponding oxirane compound. 
     Hydrazine or a halide salt thereof was used to catalyze the reaction by the patentees in U.S. Pat. No. 3,535,341 at temperatures of 100°-250° C. An anion exchange resin containing quaternary ammonium groups was disclosed in U.S. Pat. No. 4,233,221 as useful for vapor-phase reaction. 
     Organic antimony halides were shown in published Japanese patent application 80-122,776 to make possible the formation of alkylene carbonates, at room temperature to 120° C., in a water-free mixture. The time required in the single example carried out at room temperature was about 5 days, a generally impractical period of time. 
     I have now discovered that the reaction of alkylene oxides to the corresponding carbonates can be carried out with known catalysts at lower temperatures than heretofore used in the art. Further, the reaction is operable even in the presence of substantial amounts of water. The hydrolysis of the carbonates to glycols can be minimized and the principal product is the carbonate, as will be seen in the following discussion. 
     SUMMARY OF THE DISCLOSURE 
     Alkylene oxides may be reacted with carbon dioxide to form alkylene carbonates in the presence of a catalytic amount of suitable catalysts at relatively low temperatures in the range of about 20°-90° C. and in the presence of water. Preferably the temperature will be about 30°-70° C. The pressure at which the reaction is carried out is in the range of about 25-200 kg/cm 2  gauge, and may be autogenerated. The mol ratio of carbon dioxide to alkylene oxide is in the range of 1/1-100/1 and preferably about 2/1-10/1. Suitable catalysts include a member or members of the group consisting of quaternary organic ammonium and phosphonium halides, organic sulfonium halides, and organic antimony halides, particularly methyl triphenyl phosphonium iodide, tetra-ethyl ammonium bromide, and tetraphenyl antimony bromide. The corresponding carboxylates may also be used. The quantity of catalyst used is generally within the range of 0.01 to 0.15 mols per mol of alkylene oxide, preferably 0.02 to 0.10. 
     Contrary to previous expectations, water may be present in substantial amounts, even exceeding those used in prior art hydration processes, but without formation of significant amounts of glycol. Useful mol ratios of water to alkylene oxide are 0.1/1-20/1. With certain catalysts, the effect of water actually is to improve the selectivity of the conversion of the oxirane to the carbonate. 
     In another embodiment, the invention comprises a process for reacting alkylene oxides with carbon dioxide to form alkylene carbonates at relatively low temperatures in the range of about 20°-90° C. in the presence of at least one catalyst selected from the group consisting of quarternary organic ammonium and phosphonium halides and carboxylates and organic sulfonium halides and carboxylates. Such catalysts have hitherto been employed at operating temperatures higher than those now found to be useable. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Heretofore, those familiar with the reaction of alkylene oxides with carbon dioxide to form alkylene carbonates have carried out the reaction at temperatures generally in the range of 100°-300° C., particularly about 150°-225° C. Although it was not generally discussed in detail, it will be seen from prior art disclosures that the reaction was carried out in the practical absence of water. For Example, in U.S. Pat. No. 4,233,221 the reactants were dried by condensation of water after compression so that the moisture level of the reactant gases was quite low, estimated to be about 0.2 mol percent. Since the hydrolysis of carbonates was known to take place at elevated temperatures and with catalysts also useful for the direct hydrolysis of alkylene oxides to glycols, it seems probable that prior workers in the art avoided water if only the carbonate was to be produced. Otherwise, hydrolysis to the glycol could be expected. 
     Surprisingly, I have found that when the reaction of alkylene oxides with carbon dioxide is undertaken at temperatures substantially lower than taught by the prior art, that the presence of water can be not only tolerated, but actually may be beneficial in some instances. Alkylene carbonates can be prepared with minimal losses by hydrolysis to the glycols. This process is particularly useful when applied to a stream combining carbon dioxide, ethylene oxide, and water obtained by extraction of ethylene oxide from a dilute aqueous solution with near-critical or supercritical carbon dioxide. 
     The reaction may be carried out at temperatures in the range of about 20°-90° C., preferably 25°-80° C., especially 30°-70° C. Although lowering the temperature would be expected to increase the reaction time, nevertheless reasonable periods in the general range of 2-8 hours may be achieved by properly selecting the amount and type of catalyst and the other reaction conditions. In one embodiment, the temperature used is established by the ambient conditions available for cooling the feed mixture and thus would be in the range of about 20°-30° C. 
     Pressure is not an especially critical variable in the reaction. Typically, it will be in the range of about 25-200 kg/cm 2  gauge and if the temperature is sufficiently low, will be autogenerated and thus established by the feed composition and the temperature at which the reaction is carried out. 
     The molar ratio of carbon dioxide to alkylene carbonate may range from 1/1 to 100/1. Usually a ratio greater than 1/1 would be selected, preferably 2/1 to 10/1. Where the process of the invention is associated with the extraction of alkylene oxide by (near) super-critical carbon dioxide the ratio may be 40/1-60/1, with satisfactory results obtained. 
     It is of particular importance that water does not appear to hydrolyze alkylene carbonates to the glycols to a significant extent under conditions found suitable for the process of the invention. At higher temperatures, the amount of glycols produced would be expected to increase until eventually the process would no longer produce carbonates, but glycols, instead as disclosed in the patents mentioned earlier. No limit has been established on the amount of water which can be tolerated, in fact, amounts well in excess of those useful for the direct hydration of alkylene oxides to glycols have been demonstrated, as will be seen in subsequent examples. Remarkably, it has been discovered that the presence of water may have a beneficial effect on the selectivity of the reaction to produce carbonate, contrary to what might be expected. This effect may be more pronounced in association with certain catalysts, particularly those in which the bond between the halide atom and the rest of the molecule is ionic, rather than covalent in nature. 
     The catalysts found useful in the process of the invention include many of those known in the art, but used now under significantly altered conditions. Broad classes of compounds which may be useful include one or more members of the group consisting of organic quaternary ammonium or phosphonium halides organic sulfonium halides, and organic antimony halides. The corresponding carboxylates also may be used. Examples of compounds which may be employed are the following ammonium compounds tetraethyl ammonium bromide, and tetra ethyl ammonium iodide. Specific phosphonium compounds include methyl triphenyl phosphonium iodide and methyl triphenyl phosphonium bromide. Sulfonium compounds may include trimethyl sulfonium iodide and trimethyl sulfonium bromide. Antimony compounds have been found quite effective when no water is present, but appear to be adversely affected when water is included. Typical compounds are tetraphenyl antimony bromide and triphenyl antimony dichloride. Particularly preferred catalysts when water is present are methyl triphenyl phosphonium iodide and tetraethyl ammonium bromide. Of the halides, bromides and iodides are preferred. 
     The amount of catalyst will be similar to that used in other processes, about 0.01-0.15 mols of the catalyst per mol of alkylene oxide may be used, preferably 0.02-0.1 mols per mol, although larger or smaller amounts are not intended to be excluded. 
     While other workers in the field have indicated that relatively high temperatures of 100° C. or preferably more would be used either to form alkylene carbonates when no water was present, or alkylene glycols when water was available to hydrolyze alkylene oxides, it has been found that by carrying out the reaction at low temperatures in the range of about 20°-90° C., preferably 30°-70° C., one can produce alkylene carbonates and even in the presence of water. 
     The reaction to form carbonates may be carried out in the presence of substantial amounts of water. At higher temperatures typical of the prior art, glycols would be expected when water is present and, in fact, this is the basis for several processes as previously discussed. As will be seen, by operating at relatively low temperatures, it is possible to minimize hydrolysis and to form carbonates instead. 
     EXAMPLE 1 
     A sample of the catalyst being tested is introduced to a 130 cc bomb produced by the Parr Instrument Company. Samples of ethylene oxide and carbon dioxide are charged at -78° C. by immersing the bomb in a dry-ice/ acetone bath. The bomb is then closed and placed in a 36° C. bath so that the internal temperature of the bomb is increased to 30° C. and the reaction proceeds. Agitation is via a magnetically driven disk. After a suitable period of time, the bomb is removed from the bath and the contents analyzed. The results of a number of such tests are shown in Table A below. 
     
                                           TABLE A                                 
__________________________________________________________________________
Test                                                                      
   Feed, millimols                                                        
           Catalyst,                                                      
                Bath                                                      
                   Time,                                                  
                       Max. Pressure                                      
                               EO*  EC**                                  
No.                                                                       
   EO* CO.sub.2                                                           
           gms***                                                         
                °C.                                                
                   hrs kg/cm.sup.2 gauge                                  
                               Conv. %                                    
                                    Sel. %                                
__________________________________________________________________________
1  13.6                                                                   
       681 a 0.1456                                                       
                36 19.5                                                   
                       28.1    51.7 --                                    
2  13.6                                                                   
       681 b 0.4385                                                       
                36 19.5                                                   
                       29.5    55   16                                    
3  18.2                                                                   
       681 c 0.3654                                                       
                32 19.5                                                   
                       18.3    94   88.2                                  
4  15.9                                                                   
       681 d 0.3064                                                       
                37 18.5                                                   
                       15.5    37.7 --                                    
5  18.2                                                                   
       1022                                                               
           e 0.3881                                                       
                38 19.5                                                   
                       30.6    51.1 --                                    
6  22.7                                                                   
       1022                                                               
           f 0.2406                                                       
                38 19.5                                                   
                       78.1    77.9 50.4                                  
__________________________________________________________________________
 *EO = ethylene oxide                                                     
 **EC = ethylene carbonate                                                
 ***a = trimethyl sulfonium iodide                                        
 b = methyl triphenyl phosphonium iodide                                  
 c = tetraphenyl antimony bromide                                         
 d = triphenyl antimony dichloride                                        
 e = methyl triphenyl phosphonium bromide                                 
 f = tetraethyl ammonium bromide                                          
 
    
     It has been discovered that water may be present without formation of significant amounts of glycols, provided that the temperature is sufficiently low. Surprisingly, it has been found that water has a beneficial effect on the selectivity to the carbonate with some catalysts, while with others the selectivity appears to be suppressed. 
     EXAMPLE 2 
     Effect of Water on Catalysts 
     The procedure of Example 1 is followed except that varying amounts of water are introduced to the Parr bomb, with the following results. 
     
                                           TABLE B                                 
__________________________________________________________________________
Test                                                                      
   Feed, millimols                                                        
             Catalyst,                                                    
                  Bath                                                    
                     Time,                                                
                         Max. Pres.                                       
                                 EO   EC                                  
No.                                                                       
   EO  CO2                                                                
          H2O                                                             
             gms* °C.                                              
                     hrs kg/cm2 gauge                                     
                                 Conv. %                                  
                                      Sel. %                              
__________________________________________________________________________
 7 19.3                                                                   
       1022                                                               
          -- c 0.5507                                                     
                  33 19.5                                                 
                         27.8    91.1 88.5                                
 8 18.2                                                                   
       1022                                                               
          10 c 0.5466                                                     
                  34 19.5                                                 
                         27.4    95.6 47.1                                
 9 20.4                                                                   
       1022                                                               
           5 b 0.4380                                                     
                  37 21  44.3    86   35                                  
10 20.4                                                                   
       1022                                                               
          20 b 0.4227                                                     
                  37 21  42.5    92   47.4                                
11 20.4                                                                   
       1022                                                               
          40 b 0.4319                                                     
                  37 21  43.2    93   63                                  
12 22.7                                                                   
       1022                                                               
          80 b 0.4365                                                     
                  37 21  41.1    83.8 76                                  
__________________________________________________________________________
 *c = tetraphenyl antimony bromide                                        
 b = methyl triphenyl phosphonium iodide                                  
 
    
     The data of Table B show that the presence of water appears to have no recognizable effect on the overall conversion of ethylene oxide, the selectivity to ethylene carbonate is reduced when catalyst &#34;c&#34; is used, while when catalyst &#34;b&#34; is employed the selectivity to ethylene carbonate is surprisingly improved. Catalyst &#34;c&#34; would be more suitable for a reaction system in which the amount of water present is not large. Note that the ratio of water to ethylene oxide is about 0.55/1 compared to the theoretical ratio of 1/1 for the hydrolysis reaction. Catalyst &#34;b&#34; appears less effective when no water is present (see test 2) but its performance is enchanced when water is used. Note that the ratios for this catalyst shown reach nearly 4/1 water/EO. 
     Although the process of the invention is particularly useful in connection with the formation of ethylene carbonate, it is more widely applicable to other oxirane compounds, as will be seen in the following example. 
     EXAMPLE 3 
     A sample of the catalyst being tested and water (if used) is introduced to a 130 cc Parr bomb. Samples of propylene oxide and carbon dioxide are charged at -78° C. by immersing the bomb in a dry-ice/acetone bath. The bomb is then closed and placed in a 36° C. bath so that the internal temperature of the bomb is increased to 30° C. and the reaction proceeds. After a suitable period of time, the bomb is removed from the bath and the contents analyzed. The results of a number of such tests are shown in Table C below. 
     
                                           TABLE C                                 
__________________________________________________________________________
Test                                                                      
   Feed, millimols                                                        
             Catalyst,                                                    
                  Bath                                                    
                     Time,                                                
                         Max. Pres.                                       
                                 PO   PC**                                
No.                                                                       
   PO* CO2                                                                
          H2O                                                             
             gms***                                                       
                  °C.                                              
                     hrs kg/cm2 gauge                                     
                                 Conv. %                                  
                                      Sel. %                              
__________________________________________________________________________
13 20.3                                                                   
       1022                                                               
          -- a 0.5511                                                     
                  35 21  42.6    96.2 90.3                                
14 20.6                                                                   
       1022                                                               
          -- b 0.4385                                                     
                  36 21  34.2    60.2 25.4                                
15 20.2                                                                   
       1022                                                               
          -- c 0.2401                                                     
                  36 21  56.1    85.0 58.3                                
16 20.4                                                                   
       1022                                                               
           5 b 0.4382                                                     
                  36 21  47.3    88.2 34.6                                
17 20.6                                                                   
       1022                                                               
          20 b 0.4378                                                     
                  36 21  52.8    89.7 56.4                                
18 20.1                                                                   
       1022                                                               
          40 b 0.4386                                                     
                  36 21  54.2    87.4 68.3                                
19 20.4                                                                   
       1022                                                               
          80 b 0.4391                                                     
                  36 21  62.1    91.4 82.3                                
__________________________________________________________________________
 *PO = propylene oxide                                                    
 **PC = propylene carbonate                                               
 ***a = tetraphenyl antimony bromide                                      
 b = methyl triphenyl phosphonium iodide                                  
 c = tetraethyl ammonium bromide                                          
 
    
     EXAMPLE 4 
     The experimental procedure of Example 3 was followed with 1,2-butylene oxide charged in lieu of propylene oxide. The results of a number of such tests are shown in Table D below. 
     
                                           TABLE D                                 
__________________________________________________________________________
Test                                                                      
   Feed, millimols                                                        
             Catalyst,                                                    
                  Bath                                                    
                     Time,                                                
                         Max. Pres.                                       
                                 BO   BC**                                
No.                                                                       
   BO* CO2                                                                
          H2O                                                             
             gms***                                                       
                  °C.                                              
                     hrs kg/cm2 gauge                                     
                                 Conv. %                                  
                                      Sel. %                              
__________________________________________________________________________
20 20.4                                                                   
       1022                                                               
          -- a 0.5513                                                     
                  36 21  44.8    92.1 87.6                                
21 20.7                                                                   
       1022                                                               
          -- b 0.4378                                                     
                  36 21  47.2    58.3 22.6                                
22 20.1                                                                   
       1022                                                               
          -- d 0.2436                                                     
                  36 21  42.6    82.0 53.2                                
23 20.1                                                                   
       1022                                                               
           5 b 0.4386                                                     
                  36 21  51.8    89.2 40.5                                
24 20.8                                                                   
       1022                                                               
          20 b 0.4392                                                     
                  36 21  43.8    91.4 59.8                                
25 20.2                                                                   
       1022                                                               
          40 b 0.4369                                                     
                  36 21  56.2    93.4 72.8                                
26 20.6                                                                   
       1022                                                               
          80 b 0.4381                                                     
                  36 21  53.6    90.8 84.3                                
__________________________________________________________________________
 *BO = 1,2butylene oxide                                                  
 **BC = 1,2butylene carbonate                                             
 ***a = tetraphenyl antimony bromide                                      
 b = methyl triphenyl phosphonium iodide                                  
 c = tetraethyl ammonium bromide                                          
 
    
     EXAMPLE 5 
     A sample of the catalyst being tested, along with H 2  O and solvents (when used) is introduced to a 300 cc electrically heated stainless steel autoclave equipped with impeller agitation produced by Autoclave Engineers, Inc. Samples of ethylene oxide and carbon dioxide are charged at -78° C. while the autoclave is immersed in a dry-ice/acetone bath. The autoclave is then closed and heated to the desired reaction temperature. After a suitable period of time, the autoclave is cooled and the contents analyzed. The results of a number of such tests are shown in Table E below. 
     
                                           TABLE E                                 
__________________________________________________________________________
Test                                                                      
   Feed, millimols                                                        
                 Catalyst,                                                
                      Bath                                                
                         Time,                                            
                             Max. Pres.                                   
                                     EO   EC****                          
No.                                                                       
   EO* CO.sub.2                                                           
          H.sub.2 O                                                       
             THF**                                                        
                 gms***                                                   
                      °C.                                          
                         hrs kg/cm.sup.2 gauge                            
                                     Conv. %                              
                                          Sel. %                          
__________________________________________________________________________
27 347 1590                                                               
          346                                                             
             1262                                                         
                 a 20.0                                                   
                      60 4   52.0    95.6 97.0                            
28 695 2794                                                               
          695                                                             
             --  a 20.0                                                   
                      60 6   104.8   95.8 90.5                            
29 1157                                                                   
       2113                                                               
          583                                                             
             --  a 20.0                                                   
                      50 6   57.7    98.2 95.5                            
30 349 1590                                                               
          350                                                             
             1263                                                         
                 a 20.0                                                   
                      70 2   57.3    99.5 96.0                            
__________________________________________________________________________
 *EO = ethylene oxide                                                     
 **THF = tetrahydrofuran                                                  
 ***a = methyl triphenyl phosphonium iodide                               
 ****EC = ethylene carbonate